Heretofore, various types of fuel injectors and fuel injection systems have been known to effectively inject fuel into an engine cylinder of an internal combustion engine. Of the many types of fuel injection systems, the present invention is directed to unit fuel injectors, wherein a unit fuel injector is associated with each cylinder of an internal combustion engine and each unit injector includes a pump plunger driven by an associated drive train for pressurizing low pressure fuel in the injector to a high pressure and injecting fuel into each cylinder on a cyclic basis. Normally, the drive train of each injector is driven from a rotary mounted camshaft operatively driven from the engine crankshaft for synchronously controlling each unit injector independently and in accordance with the engine firing order.
One such unit fuel injector is an "open nozzle" type fuel injector wherein fuel is metered into a metering chamber open to the engine cylinder by way of the injector orifices. The injector plunger is typically positioned in the bore forming the metering chamber so that inward movement of the plunger causes compression and injection of the metered fuel.
As the need for higher engine efficiency and pollution abatement have increased, it has become increasingly evident that some economical means must be provided to vary injector timing in response to changing engine operating conditions. U.S. Pat. Nos. 4,420,116; 5,026,240; 5,209,403; 5,275,337; and 5,323,964 all provide examples of open nozzle injectors capable of varying the timing of injection. In particular, these patents disclose the use of a collapsible hydraulic link to selectively change the effective length of the cam operated fuel injector plunger. These injectors include a multi-piece plunger between which is formed a variable length timing chamber which expands and collapses during each injection cycle to control the length of the hydraulic link.
In the above mentioned open nozzle injectors, the end of fuel injection is caused by the abutment of the inner end of the plunger against the inner surface of the nozzle housing adjacent the orifices. A sharp end to injection, without subsequent secondary injection, is desired in order to minimize smoke and unburned hydrocarbons in the exhaust. Secondary injection may be minimized by pressing the plunger against a seat in the injector cup adjacent the orifices with sufficient force to expel virtually all the remaining fuel from the metering chamber. Also, the plunger must be held tightly against its seat with a sufficient hold down force necessary to prevent exhaust gases from entering the injector during the exhaust stroke of the engine. Such blow-back gases cause undesirable fouling and wear in the injector and adversely affect subsequent metering and injection.
The hold down force is applied to the plunger by the injector drive train. At the end of injection, with the inner plunger of the injectors disclosed in the aforementioned patents, in the innermost position against the cup, the timing chamber fully collapses until the outer plunger abuts the inner plunger. The magnitude of the hold down force on the inner plunger is dependent on the positioning and setting of each of the components of the drive train, including the injector plungers, drive links, cam, etc., relative to one another. Therefore, use of these injectors requires the injector train for each injector to be precisely set upon installation to create the proper hold down force thus undesirably adding to installation time and costs. Also, the particular setting of each injector necessary to obtain the desired force is likely to be different for each injector since the size tolerances of each component will vary. Moreover, throughout operation of the injector, wear between the drive train components causes the hold down force to decrease as the clearances between components becomes greater. Consequently, these open nozzle injectors require costly periodic adjustments to the drive train to reset the hold down force to compensate for injector train wear.
U.S. Pat. Nos. 4,410,137; 4,410,138; and 4,420,116, commonly assigned to the assignee of the present application, each disclose open nozzle unit fuel injectors having a variable volume timing chamber which collapses after the inner plunger reaches the innermost position (FIG. 3). This design creates a minimal hydraulic link which automatically compensates for wear in the injector actuation train. However, the minimal hydraulic link is only formed for a very short period during the beginning of the hold down period after the inner plunger has reached the cup. The timing chamber collapses soon thereafter at the beginning of the hold down period causing the outer plunger to contact the inner plunger. The outer base circle of the cam, having a constant diameter, is supposed to then operate to hold the outer plunger against the inner plunger with sufficient force. However, as discussed hereinabove, the load induced by the cam in this injector will vary with injector train wear.
Consequently, there is a need for an open nozzle fuel injector assembly capable of maintaining an optimum hold down force on the injector plunger throughout operation of the injector by compensating for injector actuator train wear without the need for expensive and time consuming injector train load setting and adjustment procedures.